1. Field of the Invention
This invention relates to MIS photodetectors and, more specifically, to such detectors showing a high quantum efficiency in the infrared range.
2. Description of the Prior Art
MIS photodetectors are known in the prior art and are normally fabricated with a first semiconductor substrate layer, preferably of mercury cadmium telluride, though other semiconductor materials can be used, a transparent layer thereon, preferably of zinc sulfide, silicon dioxide or an oxide of the substrate material, though other transparent oxides can also be used and a thin top layer of transparent metal with a thick lattice structured metal thereover. In the prior art, the transparent metal utilized has preferably been a layer of nickel having a thickness of about 60 Angstroms and the thick lattice structured metal has been about 600 Angstrom thick aluminum. It has been found that the transparent metal layer typically absorbs from about 30 to 40 percent of the infrared transmissions passing therethrough, thereby causing a loss of system sensitivity. Also, the fixed charge in the insulating layer almost always has different density beneath the thick and thin portions of the metal layer, thereby resulting in premature tunnel breakdown and lost well capacity. It is readily apparent that the infrared transmissivity of the thin metal layer will be increased by decreasing the thickness thereof. However, in practice, it has been found that with layer thicknesses less than about 60 Angstroms, device yields drop off dramatically, thereby eliminating this procedure as an option.
MIS photodetectors require the metal gate in order to control surface potential in the region of the semiconductor surface. A potential well is created for collection of charge generated by the photons which excite electrons from the valence band edge to the conduction band edge to create free charge which is to be collected and stored. The metal gate is biased relative to the bulk of the semiconductor material to create the potential well for the collection of charge that is generated by the photons which enter the semiconductor and excite the electrons.